If something is more expensive, it's probably better, right? That's the assumption most of us make. Cars, designer clothes, organic foods. It's how we think.

But what about gas? Does the store charging 10 cents a gallon more have better gas?

People have been arguing for years about whether the more expensive gas is better or whether the less expensive gas is bad for your car's engine.

Let me be clear, I'm talking about regular unleaded, not premium or super premium, just regular. The stuff we all buy.

Four or five decades ago, there was a big difference between the gas at competing stores, but not today.

"All the gas that goes through the pipelines is identical," said Jeff Lenard with the National Association of Convenience Stores. The association represents 120,000 convenience stores across the U.S. which are responsible for 80 percent of the nation's gas sales.

The only changes in the gas mixture comes at the very end when companies put in their additive packages before shipping it to the retailers. It's basically a mix of detergent used to clean the engine. That's where Chevron gets Techron and Shell gets nitrogen-enriched.

"These additive packages are proprietary," Lenard said. "Most of these different blends say they're somehow preferable based on customer studies, but all of the gas sold meets federal standards."

To put the myth to bed, there is no bad or inferior gas that will damage your car, despite what anyone says.

So how can two stations right next to each other charge such different prices for basically the same product?

Tuesday afternoon I found an AM/PM selling regular for $3.69. Just down the block, Shell was selling it at $3.97. That's a 28-cent a gallon difference.

"Fuel brands that are sold significantly less than other places in the same area, it's probably due to a business strategy," Lenard said. "If you're selling it for less than the competition, you are probably looking at making money somewhere else."

Lenard said those retailers usually rely on volume or driving people inside their stores.

The next time someone tells you to avoid Brand-X or Brand-Y because the gas is bad, you can tell them they're wrong.